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Small Engine Carburetor Troubleshooting – A beginners guide

By: Author John Cunningham. Published: 2019/01/14 at 4:03 pm

Small engine carburetors don’t handle bad or dirty gas very well; if you own a small engine, you’ll encounter carburetor problems at some point. In this guide, I’ll cover the main problems with small engine carburetors and the solutions.

So, how do you troubleshoot a small engine carburetor? The most common issue with all small engine carburetors is gumming (old gas); cleaning the carb usually solves the problem.

Typical carburetor-related problems include:

  • Tank – Outlet hole inside the tank blocks with grit stopping or slowing fuel flowing to the carburetor
  • Cap – Cap allows the tank to breathe; when the cap vent fails, it seals the tank stopping fuel flow
  • Lines – Leak at connection points and on occasion can block stopping fuel flow
  • Tap – Leak, causing fire risk
  • Fuel filter – Block or slow fuel flow to the engine
  • Pump – Fail causing a no start (Not fitted to all mowers)
  • Carburetor – Block, under fuel and over fuel causing no start or poor running
  • Fuel solenoid – Fail, stopping fuel flow (Not fitted to all mowers)
  • Intake manifold – Leak, causing engine surging
  • Dirty refueling can – Often the source of the dirt
  • Air filter – Can block, causing no start or poor running with black smoke

This guide will help you diagnose and fix your fault quickly. Although this guide covers a tractor mower carburetor repair, it will work for all small engine four-stroke engines, as they all run very similar gas systems.


Need info on a 2-stroke carburetor – check out – How does a 2-stroke carburetor work?

There are many components in the fuel system that can cause issues, but by far, the most common fault is carburetor contamination.

Very often, 5 minutes spent simply draining the gas bowl fix carburetor problems. It’s all covered here in this post, or you can check out the “Carburetor bowl draining video” and also the “Carburetor cleaning video.”
The videos walk you through the whole process – removing, stripping, cleaning, reassembly, and refitting. A good cleaning and fresh gas fix most carburetor issues.

What Is Gumming?

Basically, it’s old stale gas that turns to a sticky gel; it clogs up the tiny passages of the finely balanced carburetor. Cleaning usually solves the problem, but if it’s bad, don’t waste your time cleaning; go ahead and change out the carburetor.

How Does it Happen?

Ethanol fuel is blended with regular gas, that’s not a problem for cars, but it is for small engines. Typically, the small engine is put away for winter with the gas still in the tank. The ethanol blend attracts moisture, and the alcohol content in the gas evaporates. The result is gumming and rust – it’s a carb killer.

To prevent this from happening, I use a gas stabilizer at the end of the season mixed with the gas; it’ll keep it fresh for up to 2 years, so next spring, it’s pull and mow.  I use a product called Sta-bil gas treatment. 1 ounce treats up to 2.5 gallons; it prevents gumming and cleans the fuel system.
It can be used in all gas-powered kit including 2-stroke engines. You can use it all season; I only use it at the end of the season and when winterizing. You’ll find a link to the gas stabilizer I use here on the “Small engine repair tools” page.

Of course, not all fueling system faults are gummed-up carburetors – running some simple tests will point you in the right direction.

Need more info on the fuel system, carburetor components, and how they work, you can check them out here.

Small engine carburetor components

Gumming – Gas turns to a gel and blocks everything up – ya nasty. When it’s bad, I prefer to replace the carb. Cleaning doesn’t guarantee that you get it all, then you’re tearing it down again.

Replace – Don’t even think about it. Order a new one!

Symptoms Of Carburetor Faults

How my customers describe fuel system faults, one or more of these may sound familiar.

Customer complaints include:

  • Mower stops for no reason
  • I put the mower away for winter, and now it won’t start
  • Engine runs rough
  • Engine splutters when I cut on a slope
  • Engine dies when I start cutting grass
  • Black smoke from the muffler
  • Engine revs up and down by itself Mower only runs on choke
  • Mower blows white smoke

If any of these sound familiar, you are in the right place.

Carburetor Fault Finding

At this point, it’s assumed that you have run the Gas Shot Test and Choke System Check, and they both confirmed a fueling fault. If that is the case, your symptom will fit one of the following descriptions:

Mower won’t start; Runs rough; Blowing black smoke; Starts then dies; Surging; Lacks power; Only runs with choke; Gas leaking into the oil; Blowing white smoke; engine revving up and down by itself; Mower only runs on choke”; Mower blows white smoke.

OEM – Carburetors aren’t expensive or difficult to fit. Sometimes, it’s better to just go ahead and replace the whole unit. Carburetors do wear out, and I replace lots of them.

Fuel Solenoid

A fuel solenoid is an electromagnetic valve that simply opens as you turn on and run the mower engine. When the valve is in the open position, it allows gas into the engine.

The purpose of the valve is to close at shutdown and prevent gas leaking into the engine, which helps prevent engine run-on.

Not all mowers will have one fitted, but if you have, it will be easy to spot. It lives on the bottom of the carburetor bowl and has an electrical wire and connector fitted. 

To test the solenoid, turn the ignition on (without starting the motor), locate the sensor, and disconnect the wire; now, reconnect and listen for the click sound. If you don’t hear a click, you could have a solenoid failure or a power supply problem.

Removing the solenoid is the best way to test; that allows you to see it actually open and close. If you have a power supply problem, use a DVOM or test light to check for power.

If the solenoid fails, the mower won’t start, and a failing solenoid will cause problems like only working when it wants to, or shutting down the mower. Changing out the solenoid is easy.

Test – Remove the connector to test for the click sound, or use a test light to check for power. Briggs and Kohler’s solenoids are shown here.

Fuel Bowl Clean

In some cases, you may only need to drain the fuel bowl. In other cases, you will need to remove the carburetor and clean it thoroughly. Your carburetor may look different, but the process is the same.
In this part of the guide, I will drain just the fuel bowl and check the fuel flow. You can find your fuel bowl behind the air filter. You don’t need to remove the air filter housing to access the bowl. 

Remember, if your ethanol gas is much older than one month, it’s stale. Cleaning the bowl won’t make it go. You need to drain the tank and carburetor bowl and fill them with fresh gas.
I use the Briggs and Stratton oil extractor to remove stale gas and grit from the bottom of the gas tank; it’s easier than removing the tank. Check out the one I use here on the “Small engine repair tools” page.
If this works out, great!  If not, I wrote this guide, which will walk you through the whole process –“Remove & Clean Carburetor”.

Alternatively, it’s all covered in this video, from bowl drain and cleaning to complete carburetor removal, stripping, cleaning, rebuilding, and refitting. It’s all covered here “Carburetor cleaning video.”

Carburetor Bowl Draining

Locate – The carburetor is located behind the air filter, and you usually have enough room to work without removing any other components. Turn the gas off; if you don’t have a tap, use grips to squeeze the line.

Remove – This type of bowl doesn’t have a solenoid. Remove this bowl by removing the bowl bolt. An O-ring gasket is used to seal the bowl to the carburetor. Usually, it stays on the carburetor side, and that’s OK; you can leave it there. Clean the bowl, and when refitting, uses some lube on the o ring seal to prevent pinching.

Remove – This carburetor has a fuel solenoid. To remove it, disconnect the wire connector and use an open-ended wrench between the bowl and the solenoid.

Sometimes you can just turn the bowl by hand. Remember to lube the gasket when refitting the bowl. Often no matter how careful you are, the bowl gasket will leak gas; if so, the only fix is to replace it.

Remove – Remove the fuel bowl drain bolt, which on some models is also the fuel solenoid. Your bowl may have a bolt or two screws, and in some cases, the whole bowl will come off. Allow the fuel in the carburetor to drain out, catch it in a suitable container, and have some old rags handy. If you have any doubts about fuel quality, drain the tank and fill it with fresh gas.

Testing Fuel Flow

Flow – The carburetor bowl type with two screws can be tricky to remove, so if that’s your type, just remove the solenoid, allowing the gas to drain, reassemble and test. Often this is enough to fix the problem. But before you reassemble, check the fuel flow on whichever type of bowl you have. Turn the fuel on. If fuel flows – Refit the fuel bowl bolt and test the mower. If there is no fuel flow, we’ll need to dig a little deeper.

If you removed the fuel bowl or drain bolt and found no fuel flowing or the carburetor needle is leaking gas even with the float in the shut-off position (Up), then this guide will help you. This guide works just the same for walk-behind mowers, lawn tractors, ride-on mowers, tillers, snow blowers, or any 4-stroke small engine. 

Riding lawn mower fuel systems are either gravity feed or pump feed; your mower will be one or the other. You will be able to identify which system you have by following the fuel line from the fuel tank. Go ahead and identify your system, and carry out the checks as directed.

A weak carburetor float needle is a common problem; it causes gas to flood the engine oil. It’s known as Hydro-locking, and we’ll deal with it first before looking at identifying your fuel supply system.


Gas leaks into the cylinder when the mowers are not in use, filling it right up. This prevents the engine from cranking over because the piston has no room to move. Some owners think that the battery is flat and try jump-starting without success.

Other tell-tale signs of hydro-locking are a stink of gas in the garage, gas on the floor of the garage, mysterious loss of gas from the tank, and a very high oil level that stinks of gas.

Some mowers may start when most of the gas leaks from the cylinder into the oil. The operator then notices lots of white smoke, rough running, stalling, and oil leaks.

The fix – replace the whole carburetor because often, just replacing the needle seal doesn’t work. Fitting a gas tap and turning off the tap when the mowers are not in use will prevent future problems. But it’s important to change the engine oil; it’s diluted and contaminated by the gas.

This guide will show you how to fit a tap and the tools needed – “Fitting a gas tap.”

Identifying your fuel supply system

Gravity Fuel System – Identified by a fuel line from the tank runs to a fuel tap, through a fuel filter, and onto the carburetor. (Tap may not be fitted) This system is prone to leaking gas into the oil and causing a condition known as hydro-locking. 

Pump Fuel System – A fuel line from the tank runs to a fuel tap, then a fuel filter, then a fuel pump, and finally to the carburetor (Tap may not be fitted).

Fuel Supply Troubleshooting

Remove Gas Cap – A gas tank needs to breathe; when fuel leaves the tank, it needs to be replaced with air. A sealed tank will prevent fuel from flowing. Make sure you have gas in the tank. Remove the gas cap and check the flow. Check the fuel tank for grit – the outlet hole is small and blocks easily. You may have to remove the tank to clean it thoroughly. 

Filter – Examine the fuel lines from the tank to the carburetor, checking for kinks or damage. Some fuel filters will be a see-through bottle type; if it’s dirty – Change it. Arrow to carb.

Float in fill position

Remove – Remove the gas bowl – when the float is in the dropped position, the gas should flow.

Needle – Remove the float and needle, and check the condition. A worn needle turns pink in color. The needle seals the flow of gas when the float is in the up position. A worn needle can block the flow or cause gas to leak into the oil. When this happens, I prefer to replace the complete carburetor.

Spray – Blow some carb cleaner into the needle seat on the carburetor. Still, no flow – Remove & clean the carburetor; consider replacing the complete unit. Some carburetors have the seal on the tip of the needle, and others have the seal in the carburetor. Carb was removed for the demo.

Gas Pump – The pumped system is, as said, very similar. Check that the gas filter supply to the pump is OK. The fuel pump operates by the pulsing of crankcase pressure which is supplied by the hose pipe seen in the center of the picture. Check this pipe is secure and undamaged; sometimes, they perish.

To test the pump – Remove the output line on the left and crank over the engine. No fuel flow – Replace pump.

Remove & Clean Carburetor

Okay, I will assume you have tried cleaning the bowl as per the above guide without success. Now, you need to remove the carburetor and clean it.

Only basic tools are needed, but a can of carburetor cleaner makes life a whole lot easier. In the workshop, I use WD40 cleaner; check it out on “Small engine repair tools” page. A container for nuts and bolts, some rags, and take lots of pictures to help you remember where levers, gaskets, and springs go.

Your carburetor may not be the same as the one used here, but yours will look very similar, and the process is the same.

The whole process is covered in the “Mower surging video” and if you need to replace the carburetor, check out the Amazon carburetor link below.

Amazon Lawnmower Carburetors

Remove – Remove the air filter and engine plastic cover.

Remove – Remove the choke cable.

Turn off the gas and remove the fuel line. If you don’t have a gas tap, use grips to gently squeeze the line.

Remove gas line

Remove – Remove the intake pipe.

Remove – Unplug the solenoid valve and remove both carburetor bolts.

Remove carburetor fasteners

Photo – Take note of linkage, spring and gasket locations, and orientation before removing.


Remove – Remove the float by sliding the pinout and removing the needle. When worn, the needle seal turns pink. Carburetor kits will include new bowl gaskets and needle seals.

Remove When removing the fuel/air mix screw, count how many turns it takes to remove it and refit to the same number.

Remove – Remove the main jet with a flat screwdriver. Jets are made from brass which is a soft metal and will damage easily. Be sure the screwdriver is a good fit.

Remove – The dirt collects in the emulsion tube; it houses small portholes through which fuel flows.

Clean – Clean the jet and emulsion tube really well, the portholes may not look dirty, but a build-up around them makes them smaller and restricts gas flow. Use a strand of wire from a wire brush and run it through the holes.

Check – The bowl gasket may be distorted or perished. Over-tightening or pinching will cause it to leak. To avoid damage, lube o ring on reassembly.

Spray – Use a good quality carb cleaner and compressed air if available. Spray all passages and portholes.

OEM – A new carburetor makes a bit of difference; cleaning won’t guarantee it runs sweet. So, if cleaning doesn’t work out, go ahead and treat your mower to a new carb.

Finally – When rebuilding, replace the gas filter. Clean your gas can and fill it with fresh gas. If you’re storing the mower for periods longer than a month, use a gas stabilizer. It will prevent gumming.

Related Questions

What can a dirty carburetor cause? A dirty carburetor can have many symptoms; here’s the most common:

  • No staring
  • Lack power
  • Hesitation
  • Surging
  • Stalling

Why is my carburetor not getting gas? The most common reason a carburetor isn’t getting gas is because of a Dirty carburetor gas bowl, but there are other possible reasons:

  • Gas level too low
  • Gas tap off
  • Blocked gas tank
  • Blocked gas filter
  • Blocked float needle
  • Blocked carburetor jet
  • Blocked gas lines